Kim Bohyoung, Lee Kyoung Ho, Kim Kil Joong, Mantiuk Rafal, Kim Hye-ri, Kim Young Hoon
Seoul National University College of Medicine, Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Korea.
AJR Am J Roentgenol. 2008 Jun;190(6):W342-50. doi: 10.2214/AJR.07.3405.
The objective of our study was to assess the effects of compressing source thin-section abdominal CT images on final transverse average-intensity-projection (AIP) images.
At reversible, 4:1, 6:1, 8:1, 10:1, and 15:1 Joint Photographic Experts Group (JPEG) 2000 compressions, we compared the artifacts in 20 matching compressed thin sections (0.67 mm), compressed thick sections (5 mm), and AIP images (5 mm) reformatted from the compressed thin sections. The artifacts were quantitatively measured with peak signal-to-noise ratio (PSNR) and a perceptual quality metric (High Dynamic Range Visual Difference Predictor [HDR-VDP]). By comparing the compressed and original images, three radiologists independently graded the artifacts as 0 (none, indistinguishable), 1 (barely perceptible), 2 (subtle), or 3 (significant). Friedman tests and exact tests for paired proportions were used.
At irreversible compressions, the artifacts tended to increase in the order of AIP, thick-section, and thin-section images in terms of PSNR (p < 0.0001), HDR-VDP (p < 0.0001), and the readers' grading (p < 0.01 at 6:1 or higher compressions). At 6:1 and 8:1, distinguishable pairs (grades 1-3) tended to increase in the order of AIP, thick-section, and thin-section images. Visually lossless threshold for the compression varied between images but decreased in the order of AIP, thick-section, and thin-section images (p < 0.0001).
Compression artifacts in thin sections are significantly attenuated in AIP images. On the premise that thin sections are typically reviewed using an AIP technique, it is justifiable to compress them to a compression level currently accepted for thick sections.
本研究的目的是评估对腹部CT源薄层图像进行压缩对最终横向平均强度投影（AIP）图像的影响。
采用可逆的4:1、6:1、8:1、10:1和15:1联合图像专家组（JPEG）2000压缩比，我们比较了20组匹配的压缩薄层（0.67毫米）、压缩厚层（5毫米）以及从压缩薄层重新格式化得到的AIP图像（5毫米）中的伪影。使用峰值信噪比（PSNR）和感知质量指标（高动态范围视觉差异预测器 [HDR-VDP]）对伪影进行定量测量。通过比较压缩图像和原始图像，三位放射科医生独立将伪影评为0（无，无法区分）、1（勉强可察觉）、2（轻微）或3（明显）。采用Friedman检验和配对比例的精确检验。
在不可逆压缩时，就PSNR（p < 0.0001）、HDR-VDP（p < 0.0001）以及读者评分（6:1或更高压缩比时p < 0.01）而言，伪影在AIP图像、厚层图像和薄层图像中的增加趋势依次为AIP、厚层、薄层。在6:1和8:1时，可区分的图像对（1 - 3级）在AIP图像、厚层图像和薄层图像中的增加趋势依次为AIP、厚层、薄层。不同图像的压缩视觉无损阈值各不相同，但在AIP图像、厚层图像和薄层图像中的降低趋势依次为AIP、厚层、薄层（p < 0.0001）。
薄层图像中的压缩伪影在AIP图像中显著衰减。基于通常使用AIP技术查看薄层图像这一前提，将其压缩到目前厚层图像可接受的压缩水平是合理的。
